[Bug sanitizer/101300] New: -fsanitize=undefined suppresses -Wuninitialized

[llvm at rifkin dot dev]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101300

            Bug ID: 101300
           Summary: -fsanitize=undefined suppresses -Wuninitialized
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: sanitizer
          Assignee: unassigned at gcc dot gnu.org
          Reporter: llvm at rifkin dot dev
                CC: dodji at gcc dot gnu.org, dvyukov at gcc dot gnu.org,
                    jakub at gcc dot gnu.org, kcc at gcc dot gnu.org, marxin at gcc dot gnu.org
  Target Milestone: ---

The following code should produce a Wuninitialized warning.

int foo() {
    int n = 4;
    char b[n];
    return b[2];
}

The compiler identifies the uninitialized access when compiled -Wall however it
does not when compiled with -Wall -fsanitize=undefined.

It does warn properly when compiled with -Wall -fsanitize=undefined -O1 (or any
optimization flag other than -O0).

https://godbolt.org/z/4rrKK5MYj